Oscillator-modulator circuit



'M. J. 0. STRUTT ETAL' 2,270,791

oscmmwoa-moounmm cmcum Filed Feb. 29, 1940 -701. F. NETWORK TO A VC SOURCE R 1 m. l v m 0 L m, M. 0 I. m y I 4 A t W w o W 1T W A A 3 H fi w as:

7'0 SIGMA L COLLECTOR T0 LOCAL OSCILLATOR INVENTORS BY AND ALDERT VAN DER ZIEL T0 SIGNAL saunas Patented Jan. 20, 1942 OSCILLATOR-MODULATOR CIRCUIT Maximiliaan Julius Otto Strutt and Aldert van der Ziel, Eindhoven, Netherlands, assignors to Radio Corporation of America, a corporation of Delaware Application February 29, 1940, Serial No. 321,420

In the Netherlands July 13, 1939 8 Claims.

This invention relates to a circuit which comprises a mixing valve which is especially suited for use in a superheterodyne receiver, and also to an electron discharge tube for use in this type of circuit.

In the drawing:

Fig. 1 shows a known mixer circuit for the purpose of analysis,

Fig. 2 shows an embodiment of the invention,

Fig. 3 shows a modification.

A usual circuit in which a hexode mixing valve is used is shown in Fig. 1; numeral II designating an oscillatory circuit which is tuned to the incoming signal frequency mi. The circuit I I is connected to the first control grid I of the Valve 2. Oscillations of the frequency on, can be induced in the circuit II for example by a dipole having a coupling coil I0. I2 designates a circuit tuned to the oscillations of the frequency wh which are generated, for example, by a local oscillator and which are fed to the second control grid 3. The anode 4 is connected in the usual manner to the positive terminal of the source of voltage by way of a circuit I3 tuned to the intermediate frequency mm. The screening grids 5 and 6 are also connected in usual manner to the positive terminal of a suitable source of voltage. The resistance 1 and condenser 8 included in the cathode conductor insure the supply of a fixed negative bias to the control grid I. A variable bias e permits the grid to be more or less negative according to circumstances, for example for the purpose of automatic gain control.

When short, and more particularly ultra-short, waves are being received it is essential that the hiss resistance of the valve should be a minimum and that the damping which is exercised on the circuit II by the valve should be low. In addition, it is desirable that the parallel capacity of the valve with respect to the circuit I I should be low enough to avoid the occurrence of oscillations of the frequency 0:11 at the control grid I, and that a modification of the working point of the valve should bring about a minimum alteration of the input capacity for the control grid and for the oscillator grid. It is also essential, particularly in television circuits in which ultrahigh intermediate frequencies and wide bands are used, that the valve capacity which on the output side of the valve is caused to be in parallel with the anode circuit should be low since the impedance of the anode circuit, and thus the amplification, consequently increases with unvaried band width.

According to the present invention use is made of a mixing valve in which the oscillator grid is more remote from the cathode than the control grid, there being no screening grid between the two grids. One of the control grids consists of two halves arranged symmetrically with respect to the cathode and arranged in push-pull connection. It is thus insured that the hiss resistance is much lower than in usual constructions, and the damping due to the input capacity of the valve on either the input circuit or the oscillator circuit is much lower than in known circuit arrangements. In addition, the tube capacity which is caused to be located on the output side of the valve in parallel with the anode I circuit is much lower than in usual circuit arrangements, and any variation in the working point of the valve exercises a much lower influence on the detuning either of the input or of the auxiliary oscillatory circuit than in known circuit arrangements.

It is well known that the hiss resistance of an amplifier valve comprising one or more screening grids largely depends on the screening grid currents. It is also known to suppress screening grid currents by arranging windings of the screening grid concerned, so to say, in the shadow of the windings of the next preceding grid. Referring to Fig. 1, use is made of two screening grids viz., a grid 6 arranged between the signal control I and the oscillator grid 3 and a grid 5 arranged between the oscillator grid 3 and the anode 4. It is clear from the' above that it would be advantageous if at least one of these screening grids could be dispensed with, it being possible in this case to take the above measure for suppressing the hiss for the other screening grid.

Now, the invention is based on recognition of the fact that the screening grid 6 which primarily serves to prevent oscillations of the oscillator frequency wh from being induced on the signal control I, and from being thus enabled to be radiated by the antenna, can be dispensed with if one of the grids I or 3 is divided in two halves arranged symmetrically with respect to the cathode and arranged in push-pull connection. If, for example, as shown in Fig. 2, the oscillator grid is divided in two equal halves which are arranged in push-pull connection, the parts of the screening grid which are opposite thereto will always have voltages of equal value, but opposite direction, induced in them so that voltages of the oscillator frequency will not be conducted outwards by the screening grid. If

on the contrary, as shown in Fig. 3, the signal control I is divided in half and if the two halves are arranged in push-pull connection the two parts of this grid will always have voltages of equal value and direction induced in them by the oscillator grid so that the output circuit has not occurring in it any voltage of the oscillator frequency. Due to the omission of one of the screening grids a factor contributing. substantially to the hiss is entirely eliminated. Thus, the other screening grid may be constructed in known manner so that its windings are arranged in the shadow of the oscillator grid with the result that the influence of this grid on the hiss is also largely reduced. In practice a reduction of the equivalent hiss resistance on the input side of the valve to of the normal value can be obtained in this manner.

Fig. 2 shows one embodiment of the invention in which both the oscillator grid and the anode are divided in two parts arranged in pushpull connection. The mixing valve is designated by 2, and numeral l designates the signal control grid, 3 and 3" designate the two parts of the oscillator grid which are arranged in pushpull connection. Numeral 5 designates the single screening grid in the valve whose windings are located in the shadow of those of the two grid halves 3' and 3", and 4' and 4" designate the two anode sections arranged in pushpull connection. The same reference numerals otherwise designate like elements as in Fig. 1.

The essential features as regards operation as a mixing Valve are similar to those of the normal type: Due to the auxiliary voltage wh the mutual conductance S of the two valve halves 3'--54' and 3"5-4 is varied in the same period but in opposite sense as follows:

S1=Sg+S1 sin wht-I-Sz cos Zwht-I- S3 sin Bwhi-I- If E1 is the amplitude of the alternating voltage supplied to the grid I it is given, for example, by the formula E1 sin wt. The formula for the anode current will contain the terms S1E1 sin wit sin wht so that in the two valve halves the amplitude of the anode current having the frequency wm=wh'wi will be equal to Erm= /2 E181. The conversion conductance of such a mixing stage is, therefore, /2 81:80 and is equal to that of a usual hexode.

The fact that in Fig. 2 the oscillator Voltage wh is fed in push-pull connection shows that the cathode conductor is no longer included in the oscillator circuit. This is responsible for the damping being exercised on the circuit I 2, and, also, for the parallel capacity being largely re duced, while also the wiring capacity is substantially reduced. Due to the reduced capacity the change in capacity relative to the circuit I2 will also greatly decrease as a result of an alteration in the adjustment of the working point (for example by altering the A. V. C. bias) and this is sometimes of great importance. Due to the fact that the anode circuit is also connected in push-pull connection the resulting tube capacity which is caused to be in parallel with the circuit i3 Will be much smaller than in the usual circuit arrangement. As stated, this stands out as an advantage particularly with television receivers. Since, in addition, the anode parts are connected in push-pull connection and the input grid is not divided, a voltage of the frequency wi cannot occur in the anode circuit, which is advantageous when the intermediate frequency wh is slightly different from the frequency wi while even vice versa it is impossible for voltages of the intermediate frequency to get into the input circuit I I.

Fig. 3 shows a further embodiment of the invention, and in this case it is the input control grid which is divided in two halves l' and I". Similar reference numerals otherwise designate like parts as in Figs. 1 and 2. As before, it is possible to derive that the conversion conductance is identical with that of the usual heXode. As before, the windings of the grid 5 are arranged in the shadow of those of the grid 3. The advantages attending this embodiment as compared with a usual mixing circuit are not only a low equivalent hiss resistance, but also a low output capacity for the same reason as in the embodiment shown in Fig. 2. In addition, in this embodiment the input capacity of the valve is very small similarly to the damping caused by the tube on the input circuit II, while also the wiring capacity of the input circuit compared with normal circuit arrangements is much reduced and this permits obtaining a substantially greater amplification by means of a preceding stage.

In addition, in this embodiment the influence of a modification of the adjustment of the working point on the input capacity is appreciably lower than in Well known mixing circuits. Lastly, in the circuit arrangement shown in Fig. 3 a voltage of the oscillator frequency can cannot get into the anode circuit 13, or into the signal grid circuit or vice versa, so that many unpleasant additional phenomena that occur in the usual mixing stages are avoided. In both embodiments constructional difliculties arise when manufacturing grid halves which should be rather accurately equal to each other and symmetrical with respect to the other electrodes. According to the invention, these difficulties can be substantially reduced by at least the divided grid being built up from small rods which may be arranged, for example, in a cylindrical surface so as to surround the cathode and may be subdivided in two equal groups separately throughconnected and taken out of the envelope of the tube.

What is claimed is:

1. An oscillator-modulator circuit comprising an electron discharge tube having a cathode, a pair of control grids, a screening grid and an anode arranged successively in the order named, an input circuit tuned to the signal frequency connected to one of said control grids, a local oscillator circuit tuned to a frequency differing from the signal frequency by an intermediate frequency connected to the other of said control grids, a source of positive steady potential connected to the screening grid, and a circuit tuned to said intermediate frequency connected to the anode, one of said control grids being constructed in two halves arranged symmetrically with respect to the cathode, and the circuit associated with said latter grid being connected to the grid halves in push-pull relation.

2. An oscillator-modulator circuit comprising an electron dis-charge tube having a cathode, a .pair of control grids, a screening grid and an anode arranged successively in the order named, an input circuit tuned to the signal frequency connected to the first of said control grids, a local oscillator circuit tuned to a frequency differing from the signal frequency by an inter- .mediate frequency connected to thesecond of said control grids, a source of positive steady potential connected to the screening grid, and a circuit tuned to said intermediate frequency connected to the anode, the first of said control grids being constructed in two halves arranged symmetrically with respect to the cathode, and the input circuit associated with said latter grid being connected to the grid halves in push-pull relation.

3. An oscillator-modulator circuit according to the invention defined in claim 2, wherein the anode is constructed in two halves arranged symmetrically with respect to the cathode, corresponding halves of the first grid and the anode being arranged opposite each other, the circuit associated with said anode being connected to the anode halves in push-pull relation.

4. An oscillator-modulator circuit comprising an electron discharge tube having a cathode, a pair of control grids, a screening grid and an anode arranged successively in the order named, an input circuit tuned to the signal frequency connected to the first of said control grids, a local oscillator circuit tuned to a frequency differing from the signal frequency by an intermediate frequency connected to the second of said control grids, a source of positive steady potential connected to the screening grid, and a circuit tuned to said intermediate frequency connected to the anode, the second of said control grids being constructed in two halves arranged symmetrically with respect to the cathode, and the oscillator circuit associated with said latter grid being connected to the grid halves in pushpull relation,

5. An oscillator-modulator circuit according to the invention defined in claim 2, wherein the anode is constructed in two halves arranged symmetrically with respect to th cathode, corresponding halves of the second grid and the anode being arranged opposite each other, the circuit associated with said anode being connected to the anode halves in push-pull relation.

6. An oscillator-modulator circuit comprising an electron discharge tube having a cathode, a first control grid, a second control grid, a screen grid and an anode arranged successively in the order named, an input circuit tuned to the signal frequency connected to the first control grid, a local oscillator circuit tuned to a frequency differing from the signal frequency by an intermediate frequency connected to the second control grid, means for maintaining the screen grid at a positive potential, the windings of said screen grid being disposed in th electron shadows of the windings of the second control grid, and a circuit tuned to said intermediate frequency connected to the anode, one of said control grids and the anode being constructed in two halves arranged symmetrically with respect to the cathode, and the respective grid and anode circuits being connected to the grid and anode halves in push-pull relation.

7. An oscillator-modulator circuit according to the invention defined in claim 6, wherein the grid constructed in two halves is the first control grid to which the signal input circuit is connected.

8. An oscillator-modulator circuit according to the invention defined in claim 6, wherein the grid constructed in two halves is the second control grid to which the local oscillator circuit is connected.

MAXIMILIAAN JULIUS OTTO STRUT'I. ALDERT VAN nan ZIEL. 

